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Evolution L-318 > Lecture 24 > Flashcards

Lecture 24 Flashcards

1
Q

Eyes

A
  • optical entity that follow specific laws of physics
  • only few hundred of years is enough to go from patch of skin with photoreceptors in it to an eye with lenses
  • doesn’t take that many steps-fairly simple structure
  • evolved into diverse kinds in the cambrian radiation
  • many different kinds among animals
  • master regulatory genes shared by eyes in a different taxa
  • ongoing evolution of eye lens proteins (flexibility of genome in evolution and selection of old genes for new functions)
  • forms as an extension of the nervous system
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2
Q

Cambrian Radiation

A
  • apperance of large, fossilizable bilaterian animals
  • novel body plans and ways of development evolved
  • stabilization of body plans
  • life history evolution, including dispersible larvae
  • eyes arose many different times independently
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3
Q

Jellyfish

A
  • have eyes that point inward and can be either slits or pinholes or even lenses
  • transparent so they can see through their insides out to the other side
  • what sort of processing can they do if they don’t have a brain only a neural net
  • uses rhythm sense organs to guide themselves in moving
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4
Q

Pigment Cells

A

-keep stray light from hitting the photoreceptors

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5
Q

Pax6

A
  • everyone has this
  • important in eye development as well as nose development
  • involved in development of all known animal eyes and light sensing organs, so although distinct eyes have evolved many times vision systems are homologous at the level of basic regulatory genes
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6
Q

Embryo Cells

A

-when lens cells finally form a lens they die and for the rest of your life those cells provide the clear things you see through–>lens

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7
Q

Human Eyes vs. Squid Eyes

A
  • squid eye behind retina are photoreceptor cells and a nerve coming out of those cells and go back to the brain
  • mammal brain retina and photoreceptor cells are backward so nerves coming out go through the retina and back out the other side again and then back to the brain
  • the blind spot is wehre thenerves go back through the retina
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8
Q

Independent Evolution of an Eye

A
  • like receptor on squid

- don’t change gene but change regulatory element

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9
Q

Cnidaria

A
  • group where eyes originated
  • modern jellyfish have eyes and balance organs
  • eyes seem to look up into the bell but may help jellyfish avoid obstacles in crowded places
  • box jellies have fish-like hunting behavior and have complex eyes and other sensory organs
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10
Q

Brittle Stars

A
  • echinoderms with whole body eyes
  • entire skeleton is studded with calcite-based light-detectors
  • light sensors are part of the skeletal spicules
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11
Q

Fish Eyes

A
  • anableps can look up into air and down into water

- different properties for each set of eyes to work in a different medium

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12
Q

Specialized Eyes

A
  • can be in unexpected parts of anatomy and can have unexpected functions
  • butterfly example with eyes on genitals used for searching and copulating
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13
Q

Macropinna

A
  • upward directed tubular eyes and tiny, terminal mouth
  • visual field of highly specialized eyes does not include mouth-feeding?
  • eyes can change position from dorsally directed to rostrally directed
  • eyes contained within transparent shield that covers top of head and may provide protection for eyes from tentacles of cnidarians
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14
Q

Spider Eyes

A
  • major eyes are as good as eyes of small rodents

- high resolution

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15
Q

Vertebrate Eye

A
  • embryonic development: molecular signals from the neuroectoderm induce lens development in ectoderm
  • Pax6 function sin other functions in addition to eyes
  • Pax6 loss of function mutations are lethal in homozygotes
  • Pax6 minus has no eye or nose
  • Pax6 essential in mammals and is haplo-insufficient (# of gene copies matters)
  • heterozygotes for Pax6 have defects in eye development
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16
Q

Insect Compound Eyes

A
  • critical to hunting and avoid being caught
  • greater number of lenses, greater clarity of vision
  • dragon flies are the most effective hunters on earth; visual pursuit hunter
17
Q

Inverted Eye

A

-squid light organ built like an eye

18
Q

Genes to Make Eyes

A
  • regulatory genes in development
  • genes that play functional roles in the eye
  • the ancient genetic regulatory genes that underlie the diversity of eye evolution: Pax-6
19
Q

Pax6 In Flies

A
  • Pax6 minus eyes have reduced or missing eyes
  • turns on eye development anywhere in the body of the fly where the gene is expressed
  • individual ommatidia of ectopic eyes are normally formed but the eyes are often small and oddly shaped, reflecting absence of normal cues for eye size and shape
  • ectopic eyes don’t usually work because photoreceptor axons from misplaced eyes can’t find brain (too far to go and normal directional cues absent)
20
Q

Special Features of Some Vertebrate Eyes

A
  • improvement light gathering in the dark-role of light reflector in some eyes
  • like organs that don’t gater light, but produce it like living flash lights
  • convergent on real eyes-not evolved from eyes
21
Q

Three Properties Under Selection

A
  • properties of proteins co-opted into expression in the eye lens as “taxon-specific crystallins”
  • globular: efficient packing
  • clear: light transmission
  • stable: long term survival in lens that makes no new proteins
22
Q

Vertebrate Taxon-Specific Lens Crystallins

A
  • recruited enzymes
  • arise in many taxa
  • they represent ordinary old enzymes used at low levels of expression in a variety of metabolic processes
23
Q

Alpha Crystallin

A
  • universal in vertebrates
  • critical function in acting as a chaperonin to prevent protein degradation
  • strong selection on this one
24
Q

Beta and Gamma Crystallins

A

-widespread and may function in protein stability

25
Q

Delta Crystallin

A
  • and others are taxon specific and function in light transmission
  • tell informative stories about molecular evolution
  • weak selection on particular identities of these
26
Q

Gene Level

A
  • genes first get duplicated then acquire new expression patterns
  • still used as enzymes in original tissues but now expressed at very high levels in eye lens
  • work fine as crystallins and are selected for new second function
  • gene duplication followed by mutations that allow expression at a new site at high level–>requires changes in upstream gene regulatory sites, not changes in the coding region
  • transparent
  • stable
  • efficiently packed
27
Q

Delta Crystallin Gene Regulatory Evolution

A
  • shows that more than a single specific mutational change can lead to a common gene expression pattern in two organisms
  • flexibility in gene regulatory evolution associated with gene duplication
  • this can happen without loss of original gene function and regulation of expression
  • look at diagram
28
Q

Epsilon Crystallins

A
  • LDH (a major enzyme in glycolysis) has second role as epsilon crystallin in some eyes
  • present in some birds but not other
  • likely that frequent gene co-option events occur

Evolution L-318 (28 decks)

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